for splay leg i use the tiny bands or the tiny thin zipties and a piece of string. check every day that the haven't gotten it tighten or that they haven't grown too much, leave for 3-5 days and usually splay leg has been fixed.
However this certain case it sounds more like the chick is weak, not that its splay leg...what bedding/floor do you have it on?
However this certain case it sounds more like the chick is weak, not that its splay leg...what bedding/floor do you have it on?